Molecular fraction limits in damped Lyman α absorption systems

We have used the Green Bank Telescope (GBT) and Berkeley-Illinois-Maryland Association (BIMA) array to search for redshifted millimetre absorption in a sample of damped Lyman alpha absorption systems (DLAs). This increases the number of published systems searched from 18 to 30. In 17 cases we reach 3sigma limits of tau less than or equal to 0.1, which is a significant improvement over the previous searches and more than sufficient to detect the four known redshifted millimetre absorbers (tau greater than or similar to 1). Although the CO rotational (millimetre) column density limits obtained are weaker than the electronic (optical) limits, they may provide useful limits below the atmospheric cut-off for the Lyman and Werner H-2 bands in the UV (z(abs) less than or similar to 1.8). Using a model for the DLA metallicity evolution combined with assumed HCO+/H-2 and CO/H-2 conversion ratios, we use the molecular column density limits to calculate plausible H-2 molecular fraction limits. Finally, we use these results to discuss the feasibility of detecting rotational CO transitions in DLAs with the next generation of large radio telescopes.